The present invention generally relates to a laser driver for an optical signal transmitter in an optical communication system like a passive optical network (PON) system. More particularly, the present invention relates to a laser driver for selectively supplying output current from a laser-driving current output section by directly controlling the supply of the driving current from a driving-current-generating current source.
In recent years, optical subscriber systems are under vigorous research and development to set up a fiber-to-the-home (FTTH) communication network in the near future. However, it is economically difficult to introduce optical fibers into general home users. This is because an optical fiber has a gigantic transmission capacity, and is much more expensive than a conventional metallic communication line. Under the circumstances such as these, the PON system is expected to contribute much to the realization of the FTTH net-work considering the cost effectiveness thereof. The PON system can be less expensive, because a single optical fiber extended from a base station is branched to provide bidirectional communication service for a great number of subscribers.
A laser driver for use in an optical signal transmitter in an optical communication system selectively supplies laser-driving current responsive to a digital data signal received. A conventional laser driver has a differential configuration such as that illustrated in FIGS. 11(a) and 11(b). The laser driver with a differential configuration selectively supplies an output current by switching current paths of laser-driving current Io using a differential pair of transistors m2 and m3. In such a configuration, since constant current always flows through a power supply, small noise is generated and the switching speed of the laser driver is advantageously high. The laser driver of this type, however, is disadvantageous in that the power dissipation increases because the driving current continues to flow through a different path even in the output-disabled state as shown in FIG. 11(b).
To reduce the power dissipation, the output of the driving current may be controlled by turning ON/OFF a driving-current-generating current source transistor m1 itself as shown in FIG. 12 (see, for example, Japanese Laid-Open Publication No. 9-232635).
In such a configuration, however, the output is suspended by completely cutting off the current source transistor m1 with the gate of the transistor m1 short-circuited with the ground. Accordingly, to turn the transistor m1 ON, the gate voltage thereof should be raised by charging a large gate-source capacitance Cgs of the transistor m1 with current Is, thus causing a considerable time delay. In addition, since the time delay is variable with the current Is for charging the gate-source capacitance Cgs of the transistor m1, the delay also depends on the driving current Io after all. Furthermore, the output current shows a waveform with unsharp rising edges.
An object of the present invention is providing a laser driver that can output a current with an ideal waveform steeply rising responsive to a digital data signal without any overshoot by directly controlling the supply of the driving current from a driving-current-generating current source.
A laser driver according to the present invention includes output node, first power supply node, first switch, current generating means, current-to-voltage converting means and output-current-generating transistor. A driving current is output through the output node. A first supply voltage is applied to the first power supply node. The first switch is connected between the output node and the first power supply node. The current generating means generates a first current with a first current value while the first switch is OFF and a second current with a second current value, which is different from the first current value, while the first switch is ON. The current-to-voltage converting means converts the current supplied from the current generating means into a voltage corresponding to the current value thereof. And the output-current-generating transistor is connected between the first switch and the first power supply node and receives the voltage from the current-to-voltage converting means at the gate thereof.
In one embodiment of the present invention, the current generating means may include first and second current sources and a current path selector. The first current source supplies the first current to the current-to-voltage converting means. The second current source generates a third current with a current value representing a difference between the first and second current values. And the current path selector supplies the third current from the second current source to the current-to-voltage converting means only when the first switch is ON.
In the laser driver according to the present invention, even while the first switch is OFF (i.e., in the output-disabled state), the gate of the output-current-generating transistor is still biased with a voltage corresponding to the first current. Accordingly, when the first switch turns ON and the laser driver enters the output-enabled state, the laser driving output current rises steeply. Also, since the bias voltage applied to the gate of the output-current-generating transistor in the output-disabled state is lower than the bias voltage applied in the output-enabled state, an ideal output waveform without any overshoot can be obtained.
In this particular embodiment, the current path selector preferably includes input terminal, output terminal and second switch. The input terminal receives the third current from the second current source. The output terminal is connected to the current-to-voltage converting means. And the second switch is connected between the input and output terminals, turns ON when the first switch is ON and turns OFF when the first switch is OFF.
In the laser driver, while the first switch is OFF (i.e., in the output-disabled state), the supply of the third current from the second current source is suspended, thus reducing the power dissipation.
In another embodiment, the current generating means may include a current source and a current shunt circuit. The current source generates the second current. When the first switch is ON, the current shunt circuit supplies the second current from the current source to the current-to-voltage converting means. When the first switch is OFF, the current shunt circuit branches the second current into the first current and a third current with a current value representing a difference between the first and second current values and then supplies the first current to the current-to-voltage converting means.
In this particular embodiment, the current shunt circuit preferably includes input terminal, first and second output terminals and first and second transistors. The input terminal receives the second current from the current source. The first output terminal is connected to the first power supply node. The second output terminal is connected to the current-to-voltage converting means. The source and drain of the first transistor are connected to the input terminal and the first output terminal, respectively. The first transistor turns OFF when the first switch is ON, but makes the third current flow between the source and drain thereof when the first switch is OFF. The source and drain of the second transistor are connected to the input terminal and the second output terminal, respectively. The second transistor makes the second current flow between the source and drain thereof when the first switch is ON and makes the first current flow between the source and drain thereof when the first switch is OFF.
In the laser driver, even while the first switch is OFF (i.e., in the output-disabled state), the gate of the output-current-generating transistor is still biased with a voltage corresponding to the first current. Accordingly, when the first switch turns ON and the laser driver enters the output-enabled state, the laser driving output current rises steeply. Also, since the bias voltage applied to the gate of the output-current-generating transistor in the output-disabled state is lower than the bias voltage applied in the output-enabled state, an ideal output waveform without any overshoot can be obtained.
In still another embodiment, the current generating means may include first and second current sources. The first current source supplies the first current to the current-to-voltage converting means. The second current source supplies a third current with a current value representing a difference between the first and second current values to the current-to-voltage converting means only when the first switch is ON.
In the laser driver, while the first switch is OFF (i.e., in the output-disabled state), the second current source does not supply the third current, thus reducing the power dissipation.
In still another embodiment, the current generating means may include current source, current ratio regulator and current path selector. The current source generates the second current. The current ratio regulator branches the second current supplied from the current source into two currents with a desired current ratio and then supplies one of these two currents as the first current to the current-to-voltage converting means. The current path selector supplies the other one of the two currents that have been branched by the current ratio regulator to the current-to-voltage converting means only when the first switch is ON.
In this particular embodiment, the current ratio regulator preferably includes input terminal, first and second output terminals and first and second transistors. The input terminal receives the second current from the current source. The first output terminal is connected to the current path selector. The second output terminal is connected to the current-to-voltage converting means. The source and drain of the first transistor are connected to the input terminal and the first output terminal, respectively. The first transistor receives a first voltage at the gate thereof. The source and drain of the second transistor are connected to the input terminal and the second output terminal, respectively. The second transistor receives a second voltage at the gate thereof.
In the laser driver, the second current supplied from the current source can be branched into two currents at a desired current ratio by regulating the first and second voltages. Accordingly, the eye pattern of the optical output power can be optimized under any operating condition, thus realizing a laser driver with broadened applicability.
In still another embodiment, the current generating means may include current source, current ratio regulator, first and second current mirror circuits and current path selector. The current source generates the second current. The current ratio regulator branches the second current supplied from the current source into two currents at a desired current ratio. The first current mirror circuit receives one of the two currents branched by the current ratio regulator as an input current. The second current mirror circuit receives the other one of the two currents branched by the current ratio regulator as an input current and supplies an output current as the first current to the current-to-voltage converting means. The current path selector supplies the output current of the first current mirror circuit to the current-to-voltage converting means only when the first switch is ON.
In still another embodiment, the current ratio regulator preferably includes input terminal, first and second output terminals and first and second transistors. The input terminal receives the second current from the current source. The first output terminal is connected to the first current mirror circuit. The second output terminal is connected to the second current mirror circuit. The source and drain of the first transistor are connected to the input terminal and the first output terminal, respectively. The first transistor receives a first voltage at the gate thereof. The source and drain of the second transistor are connected to the input terminal and the second output terminal, respectively. The second transistor receives a second voltage at the gate thereof.
In the laser driver, the second current supplied from the current source can be branched into two currents at a desired current ratio by regulating the first and second voltages. Accordingly, the eye pattern of the optical output power can be optimized under any operating condition, thus realizing a laser driver with broadened applicability. In addition, since the number of cascaded transistors decreases, the laser driver can operate stably even at a lower voltage applied.
An optical transceiver according to the present invention is adapted to establish optical communication and includes a transmitter section and a receiver section. The transmitter section converts data to be transmitted into laser light by driving a laser diode and then transmits the laser light. The receiver section converts the laser light received into received data. The transmitter section includes the laser driver according to the present invention and drives the laser diode using the laser driver.
Another laser driver according to the present invention includes output node, first power supply node, gate-grounded transistor, switching transistor and output-current-generating transistor. A driving current is output through the output node. A first supply voltage is applied to the first power supply node. The gate-grounded transistor is connected between the output node and the first power supply node and receives a constant voltage at the gate thereof. The switching transistor is connected between the source of the gate-grounded transistor and the first power supply node in series to the gate-grounded transistor. The output-current-generating transistor is connected between the switching transistor and the first power supply node in series to the switching transistor. When the switching transistor is ON, the output-current-generating transistor receives a first voltage at the gate thereof. And when the switching transistor is OFF, the output-current-generating transistor receives a second voltage, which is different from the first voltage, at the gate thereof.
In the laser driver according to the present invention, even while the switching transistor is OFF (i.e., in the output-disabled state), the gate of the output-current-generating transistor is still biased with the second voltage. Accordingly, when the switching transistor turns ON and the laser driver enters the output-enabled state, the laser driving output current rises steeply. Also, since the bias voltage applied to the gate of the output-current-generating transistor in the output-disabled state is lower than the bias voltage applied in the output-enabled state, an ideal output waveform without any overshoot can be obtained.